Bright white protective laminates

ABSTRACT

Laminates of having a first outer layer of weatherable film, at least one mid layer, and a second outer layer containing an opacifying quantity of white pigment. The laminates are particularly useful for protecting photovoltaic cells, solar panels, and circuit boards. In photovoltaic cells, the laminates result in increased power generation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.11/451,574, filed Jun. 13, 2006, now U.S. Pat. No. 7,338,707, which is aContinuation-in-Part of application Ser. No. 10/411,961, filed Apr. 11,2003, now abandoned hereby incorporated in its entirety by referenceinto this application.

BACKGROUND OF THE INVENTION

The present invention relates to laminates, and more particularly tothin film laminates. Thin film laminates are useful in manyapplications, particularly where the properties of one layer of thelaminate complement the properties of another layer, providing thelaminate with a combination of properties that cannot be obtained in asingle layer film.

Photovoltaic (PV) devices are characterized by the efficiency with whichthey can convert incident solar power to useful electric power. Devicesutilizing crystalline or amorphous silicon have achieved efficiencies of23% or greater. However, efficient crystalline-based devices aredifficult and expensive to produce. In order to produce low-cost power,a solar cell must operate at high efficiency.

A number of techniques have been proposed for increasing the efficiencyand effectiveness of PV modules. One approach is to enhance lightreflection by a protective back sheet for the solar cell.

Japanese Published Patent Application No. 62-10127 suggests providing asolar cell module with a reflective back cover sheet in the form of alaminate comprising a polyester base layer and a light-reflectingaluminum coating, with the back cover sheet having a plurality ofV-shaped grooves that provide angular light reflecting facets. The cellsare spaced from one another in front of the back sheet, so that incidentlight passing through the front cover sheet and between the cells isreflected by the back cover sheet back to the transparent front coversheet.

Gonsiorawski, in U.S. Published Patent Application No. 2004-0035460,teaches production of photovoltaic modules with a back cover sheet of anionomer/nylon alloy embossed with V-shaped grooves running in at leasttwo directions and coated with a light reflecting medium.

Even though the inventions mentioned above provide a significant powerboost, the techniques described are time consuming and expensive. Thetextured material is produced in several steps. First, the film thatserves as the substrate is manufactured as a continuous or extended webhaving flat front and back surfaces, and that continuous web is thenwound onto a roll for subsequent processing. The subsequent processingcomprises first embossing the film so as to form V-shaped grooves on oneside, and then metallizing the grooved surface of the film. The film isheated so that, as it passes between the two rollers, it is soft enoughto be shaped by the ridges on the embossing roller. After formation ofgrooves, the plastic film is subjected to a metallizing process whereinan adherent metal film is formed. The metallized film is wound on a rollfor subsequent use as a light reflector means. Such a process isdescribed in Kardauskas, U.S. Pat. No. 5,994,641.

Laminates described in Kernander et al., U.S. Pat. No. 6,319,596, haveat least one outer layer of polyvinyl fluoride and a mid-layer. Suchlaminates have been used effectively in the preparation of photovoltaiccells, solar panels and circuit boards. Polyester films have been usedeffectively as a mid-layer in these laminates, alone or in combinationwith other mid-layers. Such laminates having a polyester mid-layer havebeen found to be particularly satisfactory for a variety ofapplications. However, with long-term use, the polyester film or othermid-layer can undergo some degree of degradation. Such degradationtypically results in a yellowing of the film, which, while notdetrimental to its performance characteristics, is aestheticallyundesirable.

A need accordingly remains for a laminate that exhibits high dielectricstrength, provides effective protection for the current generated in aphotovoltaic module, and which remains aesthetically satisfactory overextended use. In addition, a need exists for a protective laminate, thecomponents of which maximize the pathways for gases formed during thelamination process to escape. Particularly when such laminates are usedfor backing on photovoltaic cells, it is desirable to have a protectivestructure that will not interfere with the functioning of the cell, and,if possible, aid in the generation of power.

SUMMARY OF THE INVENTION

The present invention provides laminates that can be used in electronicdevices such as photovoltaic (PV) modules, and which satisfy the needsdescribed above. When used as protective backing sheets for PV modules,the present laminates result in an increase of the power output of themodule, maximize the pathways for gasses formed during the laminationprocess, remain aesthetically satisfactory over extended use, provideeffective protection for the current generated in the PV module andexhibit high dielectric strength.

Specifically, the present invention provides a laminate comprising

(a) a first outer layer of weatherable film;

(b) at least one mid-layer selected from at least one of the groupconsisting of (i) poly(chlorotrifluoro ethylene), (ii) polymeric filmcoated on one or both surfaces with liquid crystal polymer, (iii) liquidcrystal polymer; (iv) metal foil; and (v) polyester; and

(c) a second outer layer having a reflectivity of at least about 75.0%.

The present invention also provides articles comprising laminates asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of one embodiment of thelaminates of the present invention.

FIG. 2 is a graphical representation of the performance characteristicsof laminates of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be more fully understood by reference to theFigures and the following description. The Figures and description belowpertain to preferred embodiments of the present invention. Variationsand modifications of these preferred embodiments and other embodimentswithin the scope of the invention can be substituted without departingfrom the principles of the invention, as will be evident to thoseskilled in the art.

The first outer layer of the present laminates comprises a weatherablefilm, that is, one which withstands exposure to ultraviolet light andexposure to extreme variations of temperature and moisture. Suchmaterials include polyvinylidene fluoride (PVDF), poly pigmentedionomers, aliphatic urethanes, weatherable grade polyesters andpolyvinyl fluoride (PVF). PVF films are preferred, of which variousgrades are commercially available, including pigmented films. Ingeneral, the PVF should have a thickness of about from 25 to 75 microns.

The present laminates further comprise at least one mid-layer selectedfrom at least one of the group consisting of (i) poly(chlorotrifluoroethylene), (ii) polymeric film coated on one or both surfaces withpolyvinylidene chloride, (iii) polymeric film coated on one or bothsurfaces with liquid crystal polymer, (iv) liquid crystal polymer; (v)metal foil; and (vi) polyester. Each of these components is well knownin the art. The thickness of the mid-layer will vary with the number andindividual thickness of the components of this component, but willtypically be about from 2 to 10 mils.

The laminates of the present invention further comprise a second outerlayer having a Reflectivity of at least about 75.0%. This reflectivitycan be provided with either heavily pigmented ethylene vinyl acetate(EVA) or other polymers, taken alone or coated with light reflectingmaterial.

When pigmented EVA is used for the second outer layer, the vinyl acetatecontent of the EVA is generally about from 2 to 33 weight percent. AnEVA content of about from 2 to 8 weight percent has been found to beparticularly satisfactory, and is accordingly preferred. An importantaspect of the present invention is that this second outer layer containabout from 3 to 15 weight % white opacifying pigment. Less than about 3%has no substantial effect on power generation, while greater than about15% results in little additional benefit, and can depreciate thephysical characteristics of the film. Concentrations of about from 5 to12 weight % have been found to be particularly satisfactory, and areaccordingly preferred. The white pigment used can be selected from thosetypically used for white pigmentation, including titanium dioxide (TiO₂)and barium sulfate (BaSO₄). Of these, titanium dioxide is preferred forits ready availability. Such pigmentation can also include mica or acomponent that adds pearlescence. The white pigment facilitates thelamination process, providing pathways for the gas generated in thecourse of lamination to escape. In addition, the white pigment resultsin increased optical density and reflectivity of the laminate. This, inturn, increases the power generation of photovoltaic cells for which thelaminate is used for a protective layer.

It is known that the properties of EVA copolymer vary significantly withthe content of vinyl acetate (VA). With a VA content in EVA of up to 10wt. %, the polymer maintains the crystalline structure, is flexible andtough and its Vicat softening point is about 95° C.

When the VA content is increased, the EVA copolymer gets softer due todecreased crystallinity, the product becomes rubber elastic and theultimate tensile strength passes through a maximum. Also, the storagemodulus is decreased and the polymer becomes flexible. Between 15 and 30wt.-percent VA, the products are comparable with plasticized PVC. Theyare soft and flexible. Polymers with 30 to 40 wt. % VA are soft, elasticand highly fillable. Increasing the VA content above 40% results in asubstantially completely amorphous material. The properties of EVA withhigher levels of VA, such as heat resistance, stress crack resistance,and behavior under long-period stressing, can be significantly improvedby cross-linking. Such EVA compositions with higher VA concentrationschange from solid to a fluid at 80° C. in 2.5 minutes, becoming solidagain when EVA cross-linking starts and the temperature goes beyond 130°C.

To attain the desired reflectivity, white pigmented polyvinyl fluoride(such as that commercially available from DuPont as Tedlar® polyvinylfluoride) can be used as the second outer layer. This is preferablycoated with thin light reflecting film containing about from 20 to 60weight percent of white pigment, preferably about from 40 to 50 weightpercent of white pigment. The white pigment used can be selected, forexample, from titanium dioxide, lithopone, zinc sulfide, zinc oxide,fluorescent whitening agents, and metal flakes.

The matrix for the thin light reflecting coating can be selected from awide variety of polymers, such as acrylic polymers, urethane,polyesters, fluoropolymers, chlorofluoropolymers, epoxy polymers,polyimides, latex, thermoplastic elastomers, and ureas. The Thin lightreflecting coating can be applied to the second outer layer by any of avariety of methods known to those skilled in the art of film coatingmanufacture. Preferred methods include coating application by spraying,dipping and brushing.

The second outer layer of the laminates of the present invention canalso be prepared from chlorotrifluoroethylene-vinylidene fluoridecopolymer (CTFE/VDF), ethylene-chlorotrifluoroethylene copolymer(ECTFE), ethylene-tetrafluoroethylene copolymer (ETFE), fluorinatedethylene-propylene copolymer (FEP), polychlorotrifluoroethylene (PCTFE),perfluoroalkyl-tetrafluoroethylene copolymer (PFA),polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),tetrafluoroethylene-hexafluoropropylene copolymer (TFE/HFP),hexafluoropropylene-vinylidene fluoride copolymer (HFP/VDF),tetrafluoroethylene-propylene copolymer (TFE/P),tetrafluoroethylene-perfluoromethylether copolymer (TFE/PFMe),perfluorinated polyethers, styrenic polymers designed for use in outdoorapplications, copolymers of cycloolefines, polyurethane, polyvinylchloride and its copolymers, polyolefin homopolymers and copolymers,copolymer of ethylene and vinyl acetate, polyamides, synthetic rubberand its copolymers, silicon rubber, polyimides, polyetherimides,fluorine-containing polyimides, polycarbonates, ionomers and theircompositions, ionomer/polyamide compositions, esters and copolymers ofpolyacrylic acid, esters and copolymers of polymethacrylic acid. Ofthese, PVF, CTFE/VDF, ECTFE, PCTFE, PVDF and polyolefin homopolymers andcopolymers are preferred.

The individual layers of the laminates of the present invention can beadhesively bonded together. The specific means of forming the laminatesof the present invention will vary according to the composition of thelayers and the desired properties of the resulting laminate, as well asthe end use of the laminate.

Preferably, each of the layers is bonded together by applying anadhesive to one layer and attaching another layer, and repeating theprocess as necessary, depending on the number of layers. Variousadhesives can be used to fabricate the laminates of the presentinvention, including those presently known and used for adhering layersof other laminates together. The particular adhesive that can be usedwill vary according to the composition of the layers and the intendeduse of the laminate.

Preferred adhesives include (I) formulations comprising 600 parts byweight of polyester adhesive blend, 100 parts by weight of methylethylketone (MEK), and 100 parts by weight of toluene; and (II) formulationscomprising 535 parts by weight of polyurethane adhesive blend, 200 partsby weight of MEK, 200 parts by weight of toluene, and 0.22 parts byweight of hydrolytic stabilizer. The above preferred adhesiveformulations are both about 24% non-volatile and are typically coatedonto a layer of the laminate at about from 7 to 10 grams per squaremeter, resulting in a final adhesive layer thickness of about from 0.25mils to 0.5 mils, depending on the density of the adhesive.

In the broadest sense, fabrication of the laminates of the presentinvention typically involves four steps which can be repeated accordingto the number of layers used to form a desired laminate. These steps are(1) coating a layer of the laminate with an adhesive, typicallydissolved in a solvent carrier; (2) drying the coated layer; (3)conditioning the layer to be laminated to the coated layer; and (4)laminating the coated layer to the conditioned layer. These four stepsresult in an intermediate laminate, and the thus obtained intermediatelaminate is then processed according to the above four steps to obtain alaminate of the present invention. The above process of forming anintermediate laminate that can be used to obtain a laminate of thepresent invention can be repeated, and the number of times this processis used will vary according to the desired final product. For example, afour layer laminate of the present invention, comprising two mid-layers,can be formed by repeating the above process three times.

The coating step of the process of fabricating laminates of the presentinvention can vary, including known methods of applying laminatingadhesives to films that will form layers of a laminate. The coating canbe carried out by any conventional means, such as spray, roll, knife,curtain, or gravure coaters, or any method that permits the applicationof a uniform coating without streaks or other defects. Variations andmodifications to the coating step described herein will be apparent tothose skilled in the art, and are within the scope of the presentinvention. For all laminates of the present invention, the first step isapplying an adhesive, preferably of the type and formulation discussedabove, to the first outer layer for the laminate.

Preferably, the adhesive is applied to the first outer layer of thelaminate rather than the at least one mid-layer, because the PVFpreferred for this layer is easier to process than most of the possiblemid-layers of the present invention. Many of the mid-layers of thepresent invention, especially those formed from thin sheets of liquidcrystal polymer, can be negatively affected by repeated processingthrough the rollers used to manipulate the layer and apply the adhesive,and by the tension forces that result from such processing. In addition,in the fabrication of laminates having a second outer layer of EVA,which can be affected by the solvent used to apply the adhesive, theadhesive should be applied to the PVF layer face of the intermediatelaminate. Accordingly, because a first outer layer of PVF is stronger,more durable, and more resistant to processing than any of the possiblemid-layers, it is preferred that this layer be processed first.

According to the first step of the process of fabricating a laminate ofthe present invention, a preferred adhesive of either formulation I orII described above is applied to the first outer layer using either acomma coater or a roll applicator with a Mayer Rod metering system. Theadhesive is generally controlled to 7 to 10 grams per square meter dry.The adhesive is applied in liquid form, usually carried in a solvent.The solvents that can be used in fabricating laminates of the presentinvention include most organic solvents. Of these, MEK and toluene arepreferred.

After applying a laminating adhesive to the first outer layer asdescribed above, the coated first outer layer is dried, then passedthrough a multi zone oven to evaporate solvents from the coating. Onepossible set of oven settings for this step of the fabrication processcan be: Zone 1=120° F., Zone 2=140° F., and Zone 3=175° F. Thesesettings are typical for this phase of fabrication, especially when thedesired laminate comprises the components in Example 1 below. The dryingstep can also occur as the coated layer is passed around heated rollers.

The drying step is typically followed by conditioning the film or layerto be laminated to the first outer layer. It is preferred that the filmor layer to be laminated be conditioned while the first outer layer isbeing dried. If there is an inconsistency in the film thickness, thefilm can be heated by a series of hot rollers in order to smooth it andremove any defects, equalize any variations in thickness or formation,and otherwise improve the quality and consistency of the film.

Additional conditioning can include corona treatment according to anyknown process. Corona treatment of the film to be laminated is preferredfor CTFE and LCP mid-layer films, because this process places additionaloxygen on the surface of the film and increases surface energy toimprove the bond of the laminating adhesive, and thus improve the bondof the at least one mid-layer to the first outer layer of PVF.

After the coated first outer layer of PVF has been dried, and the atleast one mid-layer has been conditioned, the two layers are laminated.According to this process step, the two films are fed into a laminatingnip. Typically, a laminating nip comprises a heated chrome roll and arubber backing roll between which lamination takes place. Typicallaminating temperatures can be about 250-350° F., but can vary with thedesired laminate components, the adhesive used, and other factors, whichwill be evident to those skilled in the art. The laminating rollpressure, which also depends on similar variables, including theparticular films used and their thicknesses, can vary about from 50 psito 250 psi. After the layers have been laminated, the resultingintermediate laminate should be cured and can be wound for storage andin preparation for being reprocessed. The curing time and conditionswill also vary according to many factors, including the thickness of thelayers and resulting laminate, the composition of the films used toobtain the laminate, the adhesive used to bond the layers, and theenvironment in which the intermediate laminate is cured.

Subsequent laminations to form a laminate of the present invention areperformed in the same manner as described above. In embodiments wherethe second outer layer is formed from EVA, the coating process involvescoating the intermediate laminate rather than the EVA layer, because theEVA layer can be affected by the solvents that carry the adhesive.Accordingly, in such embodiments of a laminate of the present invention,it is also preferred that the adhesive be applied to the PVF side of theintermediate layer.

Line speeds for the above process will depend on the processingmachinery used, as well as the characteristics of the films used toobtain the laminate. Typical line speeds for the type of laminationprocess described above can be about 100-120 feet per minute, with adwell time of about 45 seconds. The dwell time can include the timespent in the multi zone oven, and at other stages in the fabricationprocess.

The laminates of the present invention can be formed in any dimensions,depending on the parameters of the processing equipment and theavailability and cost of component film layers having the desireddimensions. Typically, the laminates of the present invention are aboutfrom 24 to 100 inches wide. In photovoltaic applications, the desiredwidth is about from 50 to 60 inches, however, the width will typicallybe that which can be used most efficiently. For example, if there were ademand for laminate having a width of 29 inches, a laminate having awidth of 50 inches would result in unnecessary waste, and a 60 inch widelaminate would provide the most efficient dimensions.

The laminates of the present invention can be used as simple andcost-effective protective backing sheets for photovoltaic modules, asopposed to solar batteries. A battery is a device that stores energy.Even if a solar battery derives its power from its own cells, it doesnot need to function at all times, only when the battery needs power. APV module, by comparison, only generates power and sends it to a batteryor, with an inverter, to the power grid. The laminates of the presentinvention can be used in various electronic applications, most notablyas a barrier protecting the encapsulant in photovoltaic modules. Thelaminates of the present invention are resistant to breakdown effectsassociated with exposure to environmental conditions, including UV andother bands of sunlight, heat, moisture, and electrical forces. Theopacity and high concentration of white pigment in the EVA in the secondouter layer provides excellent protection for the polyester mid-layer aswell as increased permeability for gasses generated in the laminationprocess. In the use of the instant laminates for the protection ofphotovoltaic cells, the first outer layer of the laminate should bepositioned to be the outer layer of the photovoltaic cell construction.The EVA in the second outer layer provides especially good bond strengthto the encapsulants typically used for photovoltaic cells. The highconcentration of the white pigment in the second outer layer of thelaminate, when adjacent to the photovoltaic cells, also results inincreased reflectivity of the construction, which results in higherpower generation of the photovoltaic cells.

The use of the present pigmented laminates as protective backing sheetsin PV modules increases reflectivity, still leaving the surface of thecells exposed to incident light. In these applications, encapsulant isnot used. If backside encapsulant is pigmented white, during vacuumlaminating, some of that white encapsulant will flow onto the cellsurface, decreasing the incident light on the cell and reducing itspower generation.

When the present laminates are used as protective backing sheets for aPV module, the use of the laminates will result in a number ofadvantages:

-   -   1. boost the power output of the module    -   2. maximize the pathways for gases formed during the lamination        process    -   3. the module laminate will remain aesthetically satisfactory        over extended use    -   4. the laminate will provide effective protection for the        current generated in a the PV module    -   5. the laminate will exhibit high dielectric strength

According to the above general process parameters, a wide variety oflaminates of the present invention can be fabricated. The followingExamples illustrate several possible embodiments of the laminates of thepresent invention. For the sake of brevity and clarity, theseembodiments are limited to three layer laminates, however, the inventionis not limited to such laminates, and it will be clear to those skilledin the art how to repeat the fabrication process to obtain laminates ofthe present invention having more than three layers.

EXAMPLE 1 AND COMPARATIVE EXAMPLE A

In Example 1, a laminate of the present invention is prepared having afirst outer layer of PVF having a thickness of 22.5 micrometers. Anadhesive of formulation I described above is applied to the first outerlayer of PVF using a comma coater. The adhesive is controlled to 9 gramsper square meter dry. The coated first outer layer is then dried toremove the solvent. A layer of polyethylene terephthalate having athickness of 2 mils is then laminated to the PVF. The resultingintermediate laminate is then reprocessed according to the above andlaminated to a second outer layer of EVA having a vinyl acetate contentof thickness of 4 mils to obtain a laminate of the present invention.The EVA layer contained 5.5 weight % titanium dioxide pigment, and wassubstantially opaque to ultraviolet light. A cross section of thelaminate of Example 1 is shown in FIG. 1.

In Comparative Example A, the general procedure of Example 1 wasrepeated, except that the second outer layer contained no pigment.

The laminates were tested as backings for photovoltaic cells by bondingthe second outer layer to the surface of a photovoltaic cell. Thereflectivity of both Example 1 and Comparative Example A were tested,and the laminates of Example 1 were found to result in greaterreflectivity. Photovoltaic cells using the laminate of Example 1exhibited 1-1.5% higher power generation than those of ComparativeExample A.

EXAMPLES 2-5

The general procedure of Example 1 is repeated, except that themid-layer is poly(chlorotrifluoro ethylene), polymeric film coated onboth surfaces with liquid crystal polymer, liquid crystal polymer andmetal foil in Examples 2, 3, 4 and 5, respectively. If the resultinglaminates are tested as before with photovoltaic cells, similarperformance characteristics will be obtained.

Testing of the modules for electrical power output was conducted byilluminating each module with a solar simulator light source andmeasuring the short-circuit current (Isc). The performance of solarcells and modules can be described by their current vs voltagecharacteristic (I-V). The typical I-V curve is presented in FIG. 2. Thecritical parameters on the I-V curve are the open-circuit voltage (Voc),the short-circuit current (Isc) and the maximum power-point (Pmax). Isc,the maximum current at zero voltage, is directly proportional to theavailable sunlight. Voc can be determined from a linear fit to the I-Vcurve around the zero current point. Pmax is an electrical output whenoperated at a point where the product of current and voltage is atmaximum.Pmax=Imp VmpThe Fill Factor, FF, is a measure of the squareness of the I-Vcharacteristics. It is the ratio of maximum power to the product of Iscand Voc.

${F\; F} = \frac{P\;\max}{V\; o\; c\; I\; s\; c}$The power conversion efficiency, {dot over (η)}. is defined as

$\overset{.}{\eta} = {\frac{P\;\max}{P\;{in}} = \frac{F\; F\; V\; o\; c\; I\; s\; c}{P\;{in}}}$where Pin is an incident radiant power. It is determined by theproperties of the light spectrum incident upon the solar cell

The resulting I-V curve is shown 1 FIG. 2.

The module made with the TPE bright white backskin, as depicted in Table1, showed a short circuit current increase of about 3% over the modulewith TPE black backskin.

TABLE 1 Effect of pigmentation on solar cells performance Back Skin Isc,amp Reflectivity TPE White 5.22 87.8 TPT Enhanced 5.18 80.6 TPT 5.1675.4 TPE Black 5.05 4.9

1. A photovoltaic module comprising: photovoltaic cells; and a laminatebacking sheet comprising (a) a first outer layer of weatherable film;(b) at least one mid-layer bonded to the first layer selected from atleast one of the group consisting of (1) poly(chlorotrifluoro ethylene),(ii) polymeric film coated on one or both surfaces with liquid crystalpolymer, (iii) liquid crystal polymers; (iv) metal foil; and (v)polyester; and (c) a second outer layer bonded to the mid-layer whereinthe second outer layer comprises pigmented EVA with a vinyl acetatecontent of about 2-8 percent by weight and wherein the second outerlayer is substantially opaque to ultraviolet light.
 2. The photovoltaicmodule of claim 1 wherein the second outer layer of the laminate backingsheet contains at least one white opacifying pigment.
 3. Thephotovoltaic module of claim 1 wherein the laminate backing sheetfurther comprises fluorescent whitening material.
 4. The photovoltaicmodule of claim 2 wherein the second outer layer of the laminate backingsheet contains from about 3 to about 15 weight percent of whiteopacifying pigment.